Population Served: 100,000
Year Built: In October 1979, four of the local agencies that provided wastewater services in central Marin County entered into a Joint Powers Agreement: San Rafael Sanitation District, Ross Valley Sanitary District, Sanitary District No. 2 of Marin County, and the City of Larkspur. This agreement created a separate governmental entity, the Central Marin Sanitation Agency, to oversee the construction and operation of a regional wastewater facility. CMSA became operational in January 1985.
Average Daily Capacity: 10MGD ADWF
Process Description: Class V Secondary Treatment Facility
Preliminary Treatment: Focuses on odor control with a two-phase approach: calcium nitrate injection in the collection system, followed by hydrogen peroxide injection in the main interceptors before the Headworks
Primary Treatment:
Secondary Treatment:
Disinfection and Dechlorination:
Effluent Discharge:
Final effluent discharged into central San Francisco Bay via a 10,604-foot-long, 84-inch-diameter deep-water outfall with 176 duck-bill check-valve diffusers, achieving a minimum dilution of 43:1.
Solids Handling:
Energy Positive:
Jenbacher and Waukesha Cogen Engines produce 100+% of the plant’s power demand annually, utilizing biogas produced onsite by the two anaerobic digesters
“The Agency consistently exceeds federal and state regulatory requirements while protecting public health and the environment. In addition to wastewater treatment, the Agency operates an Organic Waste Receiving Facility that supports California’s efforts to divert organic diversion from landfills, reduce Greenhouse Gas Emissions, and produce renewable power. Food waste and other organic materials received are converted into renewable biogas through anaerobic digestion. The renewable biogas is used to fuel onsite cogeneration engines that help power facility operations. Since 2024, the Agency has been energy positive, generating more electricity than it consumes and exporting surplus renewable power to the local electrical grid.”
Recent Improvements:
Food 2 Energy Program: CMSA is Energy Positive. Since 2008, the Agency has been working with the local solid waste hauler, Marin Sanitary Service (MSS), on a Central Marin County commercial F2E Program. Recently, CMSA has added a partnership with Republic Services of Sonoma County. Food-to-Energy is a renewable energy program that converts food waste into fuel, which is used in a cogeneration system to produce electricity that powers Agency facilities and the treatment plant. These successful public-private partnerships have created a program with many benefits for both the environment and CMSA, including diverting food waste from the local landfill, reducing greenhouse gas (GHG) emissions, and generating additional energy at CMSA.
MSS and Republic collect pre- and post-consumer food waste from eligible restaurants, schools, and similar institutions in their service area for processing at their local transfer station. Processing involves removing contaminants such as utensils, plastic containers, bones, melon rinds, metal objects, and similar items, followed by grinding the food waste into small particles about one inch in diameter.
A dump truck delivers the cleaned and ground food waste from the MSS and Republic transfer stations to CMSA. Once at CMSA, the food waste is dumped into an underground tank, where it is mixed and processed with FOG, and then the slurry is injected into the treatment plant’s anaerobic digesters. Once in the digesters, the FOG/food waste mixture is co-digested with wastewater solids to produce additional biogas. Historically, CMSA operated the generator on biogas fuel for approximately eight hours a day, meeting the Agency’s energy needs during that period. The additional biogas generated from the FOG/food waste component enables the Agency to run the cogeneration system longer, achieve energy self-sufficiency, and, most of the time, export excess power to the grid, which is then paid for by Marin Clean Energy.
Nutrient Removal Planning: San Francisco Bay Regional Water Quality Control Board (RWB) adopted the final Nutrient Watershed Permit (Permit) in July 2024, which includes temporary and permanent limits on CMSA’s effluent to address the large harmful algal blooms that resulted in significant fish kills over the past few years. Compliance with the interim limits will be required starting in October 2024, and with the final limits by 2034.
The final limits will require an approximately 60% reduction in CMSA’s nutrient loading to the San Francisco Bay, likely necessitating significant infrastructure improvements on CMSA’s site. In anticipation of the final Permit, CMSA developed a Request for Proposal (RFP) and issued it to 10 interested consulting firms in April 2024. The RFP is for the first phase of CMSA’s Nutrient Removal Program and includes consultant services for a Nutrient Removal Alternatives Evaluation and Facilities Plan.
During FY24, CMSA met with the proposers to share background information about CMSA’s facility and answered questions about the RFP. Four proposals were received and evaluated in early FY25, and the winning proposer was selected and asked to complete the Nutrient Removal Alternatives Evaluation and Facilities Plan by the end of FY25, culminating in a recommendation of a final preferred alternative to CMSA’s Board for approval. Thereafter, CMSA anticipates completing the pre-design and final design of the preferred alternatives in FY26-28 and beginning construction in subsequent years.
CWEA is now accepting nominations for the 2026–27 Awards Program. Nominate an outstanding agency, project, team, or wastewater professional whose work deserves statewide recognition.
All nominees must be CWEA members. Some award categories begin at the Local Section level, so check with your Section for its nomination deadline and eligibility requirements.
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